The statistic that is sweeping the NHL can trace its roots back to the Italian Alps.

It was there, in the town of Varese, near the Swiss border, that goaltender Jim Corsi had what he calls an epiphany.

“Stopping pucks is not just about stopping pucks,” Corsi says. “That’s a technical area. I think the next level, the highest level of it, is understanding how to play as opposed to just reacting to the situations. That occurred somewhere along the line when a coach would say ‘anticipate the play.’ They never enlightened you with what it meant. It always sounded like ‘guess.’ Having played in Europe, because it’s an entirely different game with a bigger rink, you couldn’t survive if you didn’t understand how the game was really played.”

Corsi, now the goaltending coach for the Buffalo Sabres, had played for the Quebec Nordiques and the Edmonton Oilers, but it was during his time in Europe that he started to think differently about hockey, and specifically about the role of goaltenders. Having played between the pipes, he knew that goalies do more work than just making saves.

“In a 24-shot game, (a goaltender) may see 60 actions, but it never results in a shot,” Corsi says. “You can't just say 'that's not going to be on goal.' He'll have to react.”

It was that line of thinking that led Corsi to begin tracking shot attempts, rather than just shots on goal. Shots that miss the net, and even shots that are blocked, all require a goalie to react.

“I was trying to measure the amount of work that a goalie does,” Corsi says. “What happened along the way, this fella, an engineer in California … tied it into the work that players do, and he sorted it out in a way that reflects the work that players do. He was kind enough to say it was based on my work originally. Hence, we have this Corsi number.”

That California engineer was Gabriel Desjardins, who got his master’s degree from Berkeley and whose website, behindthenet.ca, is the nerve center of advanced statistics in hockey, built around that Corsi number. "Vic Ferrari," an pseudonymous Oilers blogger, was actually the first to use Corsi's work to track offensive production; Desjardins centralized it.

The Corsi number comes from those “actions” that the statistic’s namesake mentions. Every time a player is on the ice for a shot attempt by his team, whether it is on goal, blocked, or a miss, he gets a plus-1 Corsi. Every time he is on the ice for a shot attempt against, it is minus-1 Corsi. Generally, the numbers are only calculated at even strength, and a team’s total numbers—basically, the Corsi numbers for goaltenders—are indicative of puck possession. The Kings may have been the No. 8 seed in the Western Conference last season, but they ranked second in the NHL in Corsi, owning 54.8 percent of the total even-strength shot attempts in their games. Los Angeles struggled less with style, and more because opposing goalies had a .939 save percentage at even strength. All that possession finally paid dividends in the postseason, when they outscored their opponents 57-30 in 20 games—and 16 wins.

Broken out for individuals, Corsi numbers show how much players “tilt the ice” when they are playing. Desjardins’ website calculates Relative Corsi, which puts a player’s individual number into a team perspective by comparing his totals against what the rest of his team does when he is not playing. The leader in Relative Corsi among players appearing in at least 20 games this season is Daniel Sedin, in part because the Canucks, well known for embracing analytics, use the Swede’s line in offensively optimal situations.

Maple Leafs defenseman Jake Gardiner has an even higher Relative Corsi than Sedin, but has been limited to only 12 games this season, part of the ongoing divide between stats-loving Toronto fans and coach Randy Carlyle, who has openly pooh-poohed numbers as basic as shots on goal while his team has taken advantage of James Reimer’s hot goaltending and unsustainably high shooting percentages to clinch a playoff spot. Gardiner’s puck-possession ability for a weak puck-possession team has been pointed to by The Globe and Mail as a reason to play him more, and CBC’s Elliotte Friedman has talked about Corsi regularly on Hockey Night in Canada. That's long way from Don Cherry calling it a “dumb-dumb system” in 2010—not that Cherry has come around himself.

After Saturday’s Hockey Night in Canada game, in which the Leafs clinched a playoff spot with a 4-1 win over the Senators, Ottawa coach Paul MacLean used Corsi—without actually naming the statistic—to illustrate Reimer's importance to Toronto, saying, “Fifty shots, 27 blocked, 13 missed the net, so we had the puck a lot of the night and good opportunity to shoot it, lots of opportunity to shoot it, but obviously Reimer must be the Vezina Trophy winner and Hart Trophy winner.”

Other teams don't dance around it—many employ statistical analysts to varying degrees, Desjardins included, and much of their work begins with shot statistics.

The man whose name is on number does not worry about what it mean on a wider scale. Corsi uses statistics to help shape his practice sessions with Buffalo’s goaltenders, but he does not talk about numbers, and the statistical backing is only part of a broader coaching strategy.

“A guy sees 25 shots, but at the end of the day, he might see 75 actions at him,” Corsi says. “Where does the action come from? Where is the exposure? How often is that exposure? You really realize, hey, this goalie is getting a lot of work as opposed to just a 25-shot game. So when we devised it, I used it as a guide. It was really learning more about goaltending myself, and trying to be helpful when I would speak to goaltenders.”

Corsi’s approach is to accentuate the positive, with as much emphasis on mindset as technique—if not more. “Without a sense of humor, you’re toast,” he says. “It’s not a forgiving position.” He sees himself as more of a facilitator than a coach, and his positivity is infectious.

“He's a very good coach of mental games,” says Sabres goalie Jhonas Enroth. “He gives you good tips on how to think and how to approach a game, how to approach situations, stuff like that. That's what I like the most. He's a little bit different, for sure. He's very upbeat, a very nice guy, and he knows a lot about goaltending and life in general. That's what you have to put together to be a good goalie, I think.”

Corsi’s life in general has been a rich one. He graduated from Concordia University with an engineering degree, and during offseasons in his playing career, he put that degree to work, designing houses, buildings, and retaining walls in Montreal, Ottawa, and Quebec City. After he stopped playing, Corsi became a math and science teacher, coaching goaltending on the side until that became a full-time job—and it actually may be easier to picture Corsi at the front of a classroom than in an NHL uniform.

“Guys dressed up as him for Halloween,” says former Sabres goalie Martin Biron. “They came and dressed as Jim Corsi. Basically, you tried to look like Eugene Levy, and then you would wear a Sabres tracksuit and put one of those mustache-glasses masks on.”

Corsi says he never saw any of the Sabres who attempted the costume, but smiles as he gives his own take on what would make an effective Jim Corsi costume: “the three mustaches—one, two, three.” He points at his eyebrows before his actual mustache. “They had some good fun with it. That’s something that I like to hear. I find that if I’m teaching and the guys don’t sense that I’m interfering with their growth, but that I’m helping them with their growth, that they’re comfortable to make some fun. It’s a good thing.”

That reaction is typical of Corsi. As Biron says, “he’s always got a positive attitude about everything.” That includes the statistic, which he chuckles about and spreads the credit for. The fact that the Corsi number was developed to help explain goaltending, but now is taking on a life of its own to help explain the game on a wider scale is fitting, given the way that Corsi thinks about the position he has turned into his life’s work.